Long wavelength optical amplifier

ABSTRACT

An L band optical amplifier in disclosed. The optical amplifier includes a signal line which has an input, an output disposed optically downstream of the input, and an amplifying gain medium optically disposed between the input and the output. The optical amplifier further includes a laser optically connected to the first amplifying gain medium and an apparatus for directing C band light into the amplifying gain medium.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. ProvisionalApplication No. 60/281,167, filed Apr. 3, 2001 and No. 60/271,342, filedFeb. 23, 2001.

STATEMENT REGARDING FEDERALLY FUNDED SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.N00014-00-C-0117 awarded by the Department of the Navy.

FIELD OF THE INVENTION

The present invention relates to optical amplifiers having operatingwavelengths longer than main emission peak wavelengths, and moreparticularly to erbium doped fiber and waveguide amplifiers operating inthe long wavelength regime (1560-1620 nm), especially for wavelengthdivision multiplexing (WDM) applications.

BACKGROUND OF THE INVENTION

Conventional erbium doped fiber amplifiers (EDFA) have been extensivelyused in optical telecommunications as means to amplify weak opticalsignals in the third telecommunication window (near 1550 nm) betweentelecommunication links. Much work has been done on the design of theseamplifiers to provide efficient performance, such as high optical gainand low noise figure. However, with the recent enormous growth of datatraffic in telecommunications, owing to the Internet, intranets, ande-commerce, new optical transmission bandwidths are required to provideincreased transmission capacity in dense wavelength divisionmultiplexing (DWDM) systems.

There are a few solutions to this demand. One proposed solution is toutilize new materials compositions as a host for the fiber gain medium(instead of silica) such as telluride, which may provide broaderamplification bandwidth (up to 80 nm). However, the non-uniform gainshape and poor mechanical properties of telluride glass make theseamplifiers difficult to implement in the telecom systems. Also, Ramanamplifiers can be considered as an alternative solution to highbandwidth demand, since these amplifiers are capable of providingflexible amplification wavelength with a broad bandwidth. However, theseamplifiers place restrictions on optical system architectures because oftheir required designs for efficient performance, such as long fiberlength (>5 km), high pump power (>500 mW) and co-pumping configurations.On the other hand, relatively long erbium doped fibers (EDFs) may alsoprovide amplification in the long wavelength range (1565-1625 nm) whenthey are used with high power pump sources. This range is commonlycalled “L band”. The conventional range, also known as “C band” is inthe wavelength range between 1525-1565 nm.

In principle, L band amplifiers take advantage of the fact that EDFshave higher emission cross-section than absorption cross-section atlonger wavelengths. Therefore, for long EDFs, amplified spontaneousemission (ASE) becomes more emphasized at long wavelengths. However,there are still several issues for optimization of L band amplifiers forefficient performance. So far, reported performance of L band EDFAs hasbeen inferior to that of C band EDFAs, with drawbacks as evidenced byhigher noise figure (NF) and lower output power and gain. It would bebeneficial to provide an L band amplifier with a low noise figure andhigh output power and gain.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention provides an L band optical amplifier. Theoptical amplifier comprises a signal line including an input, an outputdisposed optically downstream of the input and an amplifying gain mediumoptically disposed between the input and the output. The opticalamplifier further comprises a laser optically connected to the firstamplifying gain medium and means for directing C band light into theamplifying gain medium, wherein the means comprises at least onereflective element optically disposed in the signal.

The present invention also provides an L band optical amplifier. Theoptical amplifier comprises a signal line including an input, an outputdisposed optically downstream of the input and an amplifying gain mediumoptically disposed between the input and the output. The opticalamplifier further comprises a laser optically connected to the firstamplifying gain medium and means for directing C band light into theamplifying gain medium, wherein the means comprises a C band seed pumpoptically connected to the signal line between the input and theamplifying gain medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate the presently preferredembodiments of the invention, and, together with the general descriptiongiven above and the detailed description given below, serve to explainthe features of the invention. In the drawings:

FIG. 1 is a schematic drawing of an L band amplifier according to afirst embodiment of the present invention.

FIG. 2 is a schematic drawing of an L band amplifier according to asecond embodiment of the present invention.

FIG. 3 is a schematic drawing of an L band amplifier according to athird embodiment of the present invention.

FIG. 4 is a schematic drawing of an L band amplifier according to afourth embodiment of the present invention.

FIG. 5 is a schematic drawing of an L band amplifier according to afifth embodiment of the present invention.

FIG. 6A is a schematic drawing of an L band amplifier according to afirst version of a sixth embodiment of the present invention.

FIG. 6B is a schematic drawing of an L band amplifier according to asecond version of the sixth embodiment of the present invention.

FIG. 6C is a schematic drawing on an L band amplifier according to athird version of a sixth embodiment of the present invention.

FIG. 7A is a schematic drawing of an L band amplifier according to afirst version of a seventh embodiment of the present invention.

FIG. 7B is a schematic drawing of an L band amplifier according to asecond version of the seventh embodiment of the present invention.

FIG. 8 a schematic drawing of an L band amplifier according to a neighth embodiment of the present invention.

FIG. 9 is a graph showing measured gain and noise figures vs. inputsignal wavelength for the sixth and seventh embodiments of the presentinvention.

FIG. 10 is a graph showing calculated gain at 1600 nm vs. amplifyingmedium length at various wavelengths of a seed signal at 0 dBm for theeighth embodiment of the present invention.

FIG. 11 is a graph showing calculated gain vs. amplifying medium lengthat various seed powers at 1560 nm for a 1600 nm−30 dBm signal for theseventh embodiment of the present invention.

FIG. 12 is a graph showing calculated noise figure vs. amplifying mediumlength for a signal with and without seed for the eighth embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, like numerals indicate like elements throughout. Thepresent invention provides novel techniques and arrangements forimproving the performance of L band EDFAs. In general, the presentinvention utilizes ASE in the C band to provide additional amplificationcapability in the amplifier. The ASE is generated during signalamplification by a conventional pump laser as a supplemental pump sourcefor L band amplification or by a separate C band seed pump.

FIG. 1 shows a schematic drawing of an L band amplifier 100 according toa first embodiment of the present invention. The amplifier 100 includesa signal line 102 which extends from an input P_(in) at one end of theamplifier 100 to an output P_(out) at another end of the amplifier 100.Preferably, the signal line 102 is constructed from a polymer, and morepreferably, from a perfluoropolymer, although those skilled in the artwill recognize that the signal line 102 can be a glass or other lighttransmitting medium, including a waveguide. The input P_(in) and theoutput P_(out) are optically connected to each other along the signalline 102 through the amplifier 100. Components are defined to be“optically connected” when light signals can be transmitted betweenthose components. Signal light λ_(S) having at least one, andpreferably, multiple wavelengths is transmitted through the amplifier100 from the input P_(in) to the output P_(out), from left to right asshown in FIG. 1. The wavelengths of the signal light λ_(S) preferablyrange approximately from 1565 to 1625 nanometers, placing the signallight λ_(S) in the L band. Those skilled in the art will recognize thatthe signal line 102 can be a fiber, a waveguide, or other lighttransmitting device.

A first optical isolator 110 is optically disposed in the signal line102 between the input P_(in) and the output P_(out). The first opticalisolator 110 prevents backscattered light and other optical noise fromtraveling backward along the signal line 102, from the first opticalisolator 110 toward the input P_(in). A C-L band multiplexer 120 isdisposed along the signal line 102 optically downstream of the firstoptical isolator 110. As used herein, the term “optically downstream” isdefined to mean a direction along the signal line 102 from the inputP_(in) toward the output P_(out). The C-L band multiplexer 120 couples afirst end of an ASE guide 122 to the signal line 102. A second end ofthe ASE guide 122 is preferably connected to a mirror 124. Preferably,the mirror 124 is made by gold deposition to maximize reflection,although those skilled in the art will recognize that other types ofmirrors can be used. Alternatively, instead of using the mirror 124, thesecond end of the ASE guide 122 can be polished to provide Fresnelreflection. Use of a gold mirror provides approximately 90% reflectionof incident light back into the ASE guide 122, while a polished guideend provides only approximately 4% reflection back into the ASE guide122. Those skilled in the art will recognize that the ASE guide 122 canbe a fiber, a waveguide, or other light transmitting device.

A pump-signal multiplexer 130 is disposed along the signal line 102optically downstream of the C-L band multiplexer 120. The pump-signalmultiplexer 130 couples a pump laser 134 to the signal line 102 via apigtail 132. Preferably, the pump laser 134 is a 980 nanometer laserwhich emits a pump signal λ_(P), although those skilled in the art willrecognize that other wavelengths can be used as well. Also preferably,the pump laser 134 has an output power of at least 100 mW, althoughthose skilled in the art will recognize that the pump laser 124 can haveother output powers as well. Although a laser 134 is preferred tooptically connect via the pigtail 132 to the pump-signal multiplexer130, those skilled in the art will recognize that other opticalconnection techniques, such as free space coupling, can be used instead.

A rare earth doped amplifying gain medium 140 is disposed along thesignal line 102 optically downstream of the pump-signal multiplexer 130.Preferably, the rare earth is erbium, although those skilled in the artwill recognize that other elements, including, but not limited tolanthanum, cerium, praseodymium, neodymium, promethium, samarium,europium, gadolinium, terbium, dysprosium, holmium, thulium, ytterbium,lutetium, and combinations and blends thereof can be used. Although theamplifying gain medium 140 does not have a minimum or maximum length,those skilled in the art will recognize that the length of theamplifying gain medium 140 can be varied, in conjunction with differentoutput powers of the pump laser 134, to provide different amplificationgains and/or output powers. While the amplifying gain medium 140 ispreferably a fiber, those skilled in the art will recognize that theamplifying gain medium 140 can also be a waveguide or other lighttransmitting device.

A second optical isolator 150 is disposed along the signal line 102optically downstream of the amplifying gain medium 140. The secondoptical isolator 150 prevents backscattered light and other opticalnoise from traveling backward along the signal line 102, from the secondoptical isolator 150 toward the amplifying gain medium 140. The secondoptical isolator 150 is optically connected to the output P_(out) of theamplifier 100.

The devices described above, including the optical isolators 110, 150,the multiplexers 120, 130, amplifying gain medium 140, and the pumplaser 134, can also be used in amplifying C band signals.

In operation, the signal light A having a wavelength band ofapproximately between 1565 and 1625 nanometers is inserted into theamplifier 100 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 102 to the first opticalisolator 110. The signal light λ_(S) passes through the first opticalisolator 110 and along the signal line 102 to the C-L band multiplexer120. The signal light λ_(S) then passes through the C-L band multiplexer120 to the pump-signal multiplexer 130.

The pump laser 134 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 132 to the pump-signal multiplexer 130. At thepump-signal multiplexer 130, the signal light λ_(S) is combined with thepump signal λ_(P) emitted by the pump laser 134. The combined signallight λ_(S) and the pump signal λ_(P) are transmitted to the amplifyinggain medium 140. The pump signal λ_(P) excites the rare earth element inthe amplifying gain medium 140, amplifying the signal light λ_(S), as iswell known in the art. The amplified signal light λ_(S) is thentransmitted from the amplifying gain medium 140, through the secondoptical isolator 150, and to the output P_(out).

However, as the signal light λ_(S) is transmitted through the amplifyinggain medium 140, ASE, which travels in both forward and backwarddirections relative to the signal light λ_(S), is generated. BackwardASE light for an L band signal is generated in the C band, withwavelengths of approximately between 1525 and 1565 nanometers. Thebackward ASE travels in a second, opposite direction from the signallight λ_(s), toward the input P_(in). The ASE travels through thepump-signal multiplexer 130 to the C-L band multiplexer 120. At the C-Lband multiplexer 120, any L band light is directed along the signal line102 to the first optical isolator 110, which blocks further transmissionof the L band light toward the input P_(in), while C band light in theform of the ASE is directed along the ASE guide 122. The ASE travelsthrough the ASE guide 122 to the mirror 124, where the ASE is reflectedback through the ASE guide 122. The ASE combines with the signal lightλ_(S) at the C-L band multiplexer 120 and is transmitted toward thepump-signal multiplexer 130. At the pump-signal multiplexer 130, the ASEand the light signal λ_(S) combine with the pump signal λ_(P). Since theASE is in the C band range, the ASE acts as a supplemental pump source,increasing the amplification capacity of the amplifier 100 in the L bandrange. The ASE has sufficient energy and proper wavelengths to pump thesignal light λ_(S) in manner similar to the pump laser 134. Afteramplification by the pump laser 134 and by the reflected ASE, the signallight λ_(S) has an amplified intensity, larger than the initialintensity.

A second embodiment of an L band amplifier 200 according to the presentinvention is shown schematically in FIG. 2. The amplifier 200 includes asignal line 202 which extends from an input P_(in) at one end of theamplifier 200 to an output P_(out) at another end of the amplifier 200.The input P_(in) and the output P_(out) are optically connected to eachother along the signal line 202 through the amplifier 200. Signal lightλ_(S) having at least one, and preferably, multiple wavelengths istransmitted through the amplifier 200 from the input P_(in), to theoutput P_(out), from left to right as shown in FIG. 2. The wavelengthsof the signal light λ_(S) preferably range approximately from 1565 to1625 nanometers, placing the signal light λ_(S) in the L band.

A first optical isolator 210 is optically disposed in the signal line202 between the input P_(in) and the output P_(out). The first opticalisolator 210 prevents backscattered light and other optical noise fromtraveling backward along the signal line 202, from the first opticalisolator 210 toward the input P_(in). A first C-L band multiplexer 220is disposed along the signal line 202 optically downstream of the firstoptical isolator 210. The first C-L band multiplexer 220 couples a firstend of an ASE guide 222 to the signal line 202. A second end of the ASEguide 222 is preferably connected to a second C-L band multiplexer 250which is disposed along the signal line 202 optically downstream of thefirst C-L band multiplexer 220.

A pump-signal multiplexer 230 is disposed along the signal line 202optically downstream of the first C-L band multiplexer 220. Thepump-signal multiplexer 230 couples a first end of a pump laser guide232 to the signal line 202. A second end of the pump laser guide 232 isconnected to a pump laser 234. Preferably, the pump laser 234 is a 980nanometer laser which emits a pump signal λ_(P), although those skilledin the art will recognize that other wavelengths can be used as well.Also preferably, the pump laser 234 has an output power of at least 100mW, although those skilled in the art will recognize that the pump laser234 can have other output powers as well.

A rare earth doped amplifying gain medium includes a first amplifyinggain portion 240 and a second amplifying gain portion 260. The firstamplifying gain portion 240 is disposed along the signal line 202optically downstream of the pump-signal multiplexer 230. The secondamplifying gain portion 260 is disposed along the signal line 202optically downstream of the first amplifying gain portion 240. Althoughthe amplifying gain portions 240, 260 do not have a minimum or maximumlength, those skilled in the art will recognize that the lengths of theamplifying gain portions 240, 260 can be varied, in conjunction withdifferent output powers of the pump laser 234, to provide differentamplification gains and/or output powers.

The second C-L band multiplexer 250 is disposed along the signal line202 optically between the first and second amplifying gain portions 240,260. A second optical isolator 270 is disposed along the signal line 202optically downstream of the second amplifying gain portion 260. Thesecond optical isolator 270 prevents backscattered light and otheroptical noise from traveling backward along the signal line 202, fromthe second optical isolator 270 toward the second amplifying gainportion 260. The second optical isolator 270 is optically connected tothe output P_(out) of the amplifier 200.

In operation, the signal light λ_(S) having a wavelength band ofapproximately between 1565 and 1625 nanometers is injected into theamplifier 200 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 202 to the first opticalisolator 210. The signal light λ_(S) passes through the first opticalisolator 210 and along the signal line 202 to the first C-L bandmultiplexer 220. The signal light λ_(S) passes through the first C-Lband multiplexer 220 to the pump-signal multiplexer 230.

The pump laser 234 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 232 to the pump-signal multiplexer 230. At thepump-signal multiplexer 230, the signal light λ_(S) is combined with thepump signal λ_(P) emitted by the pump laser 234. The combined signallight λ_(S) and the pump signal λ_(P) are transmitted to the firstamplifying gain portion 240. The pump signal λ_(P) excites the rareearth element in the first amplifying gain portion 240, amplifying thesignal light λ_(S).

However, as the signal light λ_(S) is transmitted through the firstamplifying gain portion 240, first ASE, which travels in both forwardand backward directions relative to the signal light λ_(S), isgenerated. Only backward ASE will be discussed. The first ASE travels ina second, opposite direction from the signal light λ_(S), toward theinput P_(in). The first ASE travels through the pump-signal multiplexer230 and to the first C-L band multiplexer 220. At the first C-L bandmultiplexer 220, any L band light is directed along the signal line 202to the first optical isolator 210, which blocks further transmission ofthe L band light toward the input P_(in), while C band light in the formof the ASE is directed along the ASE guide 222.

The signal light λ_(S), now amplified by the first amplifying gainportion 240, is transmitted to the second C-L band multiplexer 250,where the signal light λ_(S) is combined with the first ASE fortransmission to the second amplifying gain portion 260. Residual pumpsignal λ_(P) is combined with the first ASE to excite the rare earthelement in the second amplifying gain portion 260, further amplifyingthe signal light λ_(S). The amplified signal light λ_(S) is thentransmitted from the second amplifying gain portion 260, through thesecond optical isolator 270, and to the output P_(out).

However, as the signal light λ_(S) is transmitted through the secondamplifying gain portion 260, second ASE, which travels in both forwardand backward directions relative to the signal light λ_(S), isgenerated. Again, only backward ASE will be discussed. The second ASEtravels in the second, opposite direction from the signal light λ_(S),toward the second C-L band multiplexer 250. The second ASE travelstoward the second C-L band multiplexer 250 and to the first C-L bandmultiplexer 220. At the second C-L band multiplexer 250, any L bandlight is directed along the signal line 202 to the first opticalisolator 210, which blocks further transmission of the L band lighttoward the input P_(in), while C band light in the form of the ASE isdirected along the ASE guide 222.

A third embodiment of an L band amplifier 300 according to the presentinvention is shown schematically in FIG. 3. The amplifier 300 includes asignal line 302 which extends from an input P_(in) at one end of theamplifier 300 to an output P_(out) at another end of the amplifier 300.The input P_(in) and the output P_(out) are optically connected to eachother along the signal line 302 through the amplifier 300. Signal lightλ_(S) having at least one, and preferably, multiple wavelengths istransmitted through the amplifier 300 from the input P_(in) to theoutput P_(out), from left to right as shown in FIG. 3. The wavelengthsof the signal light λ_(S) preferably range approximately from 1565 to1625 nanometers, placing the signal light λ_(S) in the L band.

A first optical isolator 310 is optically disposed in the signal line302 between the input P_(in) and the output P_(out). The first opticalisolator 310 prevents backscattered light and other optical noise fromtraveling backward along the signal line 302, from the first opticalisolator 310 toward the input P_(in). A first C-L band multiplexer 320is disposed along the signal line 302 optically downstream of the firstoptical isolator 310. The first C-L band multiplexer 320 couples a firstend of an ASE guide 422 to the signal line 302. A second end of the ASEguide 322 is preferably connected to a second C-L band multiplexer 360which is disposed along the signal line 302 optically downstream of thefirst C-L band multiplexer 320.

A pump-signal multiplexer 330 is disposed along the signal line 302optically downstream of the first C-L band multiplexer 320. Thepump-signal multiplexer 330 couples a first end of a pump laser guide332 to the signal line 302. A second end of the pump laser guide 332 isconnected to a pump laser 334. Preferably, the pump laser 334 is a 980nanometer laser which emits a pump signal λ_(P), although those skilledin the art will recognize that other wavelengths can be used as well.Also preferably, the pump laser 334 has an output power of at least 100mW, although those skilled in the art will recognize that the pump laser334 can have other output powers as well.

A rare earth doped amplifying gain medium includes a first amplifyinggain portion 340 and a second amplifying gain portion 380. The firstamplifying gain portion 340 is disposed along the signal line 302optically downstream of the pump-signal multiplexer 330. A first980-1580 nm multiplexer 350 is disposed along the signal line 302optically downstream of the first amplifying gain portion 340. A second980-1580 multiplexer 370 is disposed along the signal line 302 opticallydownstream of the first 980-1580 multiplexer 350. A bypass guide 352optically connects the first and second 980-1580 multiplexers 350, 370.The second C-L band multiplexer 360 is optically disposed along thesignal line 302 between the first 980-1580 multiplexer 350 and thesecond 980-1580 multiplexer 370, such that the bypass guide 352optically directs the 980 nm pump light around the second C-L bandmultiplexer 360 via 980 nm ports of the 980-1580 multiplexers 350, 370.

The second rare earth doped amplifying gain portion 380 is disposedalong the signal line 302 optically downstream of the second 980-1580multiplexer 370. Although the amplifying gain portions 340, 380 do nothave a minimum or maximum length, those skilled in the art willrecognize that the lengths of the amplifying gain portions 340, 380 canbe varied, in conjunction with different output powers of the pump laser334, to provide different amplification gains and/or output powers.

A second optical isolator 390 is disposed along the signal line 302optically downstream of the second amplifying gain portion 380. Thesecond optical isolator 390 prevents backscattered light and otheroptical noise from traveling backward along the signal line 302, fromthe second optical isolator 390 toward the second amplifying gainportion 380. The second optical isolator 390 is optically connected tothe output P_(out) of the amplifier 300.

In operation, the signal light λ_(S) having a wavelength band ofapproximately between 1565 and 1625 nanometers is injected into theamplifier 300 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 302 to the first opticalisolator 310. The signal light As passes through the first opticalisolator 310 and along the signal line 302 to the first C-L bandmultiplexer 320. The signal light λ_(S) passes through the first C-Lband multiplexer 320 to the pump-signal multiplexer 330.

The pump laser 334 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 332 to the pump-signal multiplexer 330. At thepump-signal multiplexer 330, the signal light λ_(S) is combined with thepump signal λ_(P) emitted by the pump laser 334. The combined signallight λ_(S) and the pump signal λ_(P) are transmitted to the firstamplifying gain portion 340. The pump signal λ_(P) excites the rareearth element in the first amplifying gain portion 340, amplifying thesignal light λ_(S). However, as the signal light λ_(S) is transmittedthrough the first amplifying gain portion 340, first ASE is generated.The first ASE travels in a second, opposite direction from the signallight λ_(S), toward the input P_(in).

The first ASE travels through the pump-signal multiplexer 330 and to thefirst C-L band multiplexer 320. At the first C-L band multiplexer 320,any L band light is directed along the signal line 302 to the firstoptical isolator 310, which blocks further transmission of the L bandlight toward the input P_(in), while C band light in the form of thefirst ASE is directed along the ASE guide 322 to the second C-L bandmultiplexer 360. The first ASE is then transmitted to the second980-1580 nm multiplexer 370. The signal light λ_(S), now amplified bythe first amplifying gain portion 340, is transmitted through the first980-1580 multiplexer 350 to the second C-L band multiplexer 360, wherethe signal light λ_(S) is combined with the first ASE for transmissionthrough the second 980-1580 multiplexer 370 to the second amplifyinggain portion 380.

Pump light λ_(P) which exits the first amplifying gain portion 340 isdiverted by the first 980-1580 multiplexer 350 to the bypass guide 352.The pump light λ_(P) is transmitted through the bypass guide 352 to thesecond 980-1580 multiplexer 370, where the pump light λ_(P) isrecombined with the signal light λ_(S). The bypass guide 352 isinstalled between the first and second amplifying gain portions 340, 380to eliminate any high insertion loss which may occur if the pump lightλ_(P) is directed through the second C-L band multiplexer 360 whileallowing the first ASE to recycle into the second amplifying gainportion 380.

The combined pump light λ_(P) and signal light λ_(S), as well as thefirst ASE, are then transmitted to the second amplifying gain portion380. The pump signal λ_(P) and the first ASE excite the rare earthelement in the second amplifying gain portion 380, further amplifyingthe signal light λ_(S). The signal light λ_(S) that has been amplifiedin the second amplifying gain portion 380 is then transmitted along thesignal line 302 to the second optical isolator 390 and to the outputP_(out).

However, as the signal light λ_(S) is transmitted through the secondamplifying gain portion 380, second ASE, which travels in both forwardand backward directions relative to the signal light λ_(S), isgenerated. Only the backward ASE will be discussed. The second ASEtravels in the second, opposite direction from the signal light λ_(S),toward the input P_(in). The second ASE is diverted by the second C-Lband multiplexer 360, along the ASE guide 322 to the first C-L bandmultiplexer 320. The second ASE is combined with the pump light λ_(P) atthe pump-signal multiplexer 330. The second ASE enhances the pumping ofthe signal light λ_(S) in the first amplifying gain portion 340.

A fourth embodiment of an L band amplifier 400 according to the presentinvention is shown schematically in FIG. 4. The amplifier 400 includes asignal line 402 which extends from an input P_(in) at one end of theamplifier 400 to an output P_(out) at another end of the amplifier 400.The input P_(in) and the output P_(out) are optically connected to eachother along the signal line 402 through the amplifier 400. Signal lightλ_(S) having at least one, and preferably, multiple wavelengths istransmitted through the amplifier 400 from the input P_(in) to theoutput P_(out), from left to right as shown in FIG. 4. The wavelengthsof the signal light λ_(S) preferably range approximately from 1565 to1625 nanometers, placing the signal light λ_(S) in the L band.

A first optical isolator 410 is optically disposed in the signal line402 between the input P_(in) and the output P_(out). The first opticalisolator 410 prevents backscattered light and other optical noise fromtraveling backward along the signal line 402, from the first opticalisolator 410 toward the input P_(in). A first C-L band multiplexer 420is disposed along the signal line 402 optically downstream of the firstoptical isolator 410. The first C-L band multiplexer 420 couples a firstend of an ASE guide 422 to the signal line 402. A second end of the ASEguide 422 is preferably connected to a second C-L band multiplexer 426which is disposed along the signal line 402 optically downstream of thefirst C-L band multiplexer 420.

A pump-signal multiplexer 430 is disposed along the signal line 402optically downstream of the first C-L band multiplexer 420. Thepump-signal multiplexer 430 couples a first end of a pump laser guide432 to the signal line 402. A second end of the pump laser guide 432 isconnected to a pump laser 434. Preferably, the pump laser 434 is a 980nanometer laser which emits a pump signal λ_(P), although those skilledin the art will recognize that other wavelengths can be used as well.Also preferably, the pump laser 434 has an output power of at least 100mW, although those skilled in the art will recognize that the pump laser434 can have other output powers as well.

A rare earth doped amplifying gain medium includes a first amplifyinggain portion 440 and a second amplifying gain portion 480. The firstamplifying gain portion 440 is disposed along the signal line 402optically downstream of the pump-signal multiplexer 430. A first980-1580 multiplexer 450 is disposed along the signal line 402 opticallydownstream of the first amplifying gain portion 440. A second 980-1580multiplexer 470 is disposed along the signal line 402 opticallydownstream of the first 980-1580 multiplexer 450. A bypass guide 452optically connects the first and second 980-1580 multiplexers 450, 470.A second optical isolator 460 is optically disposed along the signalline 402 between the first 980-1580 multiplexer 450 and the second980-1580 multiplexer 470, such that the bypass guide 452 opticallybypasses the second optical isolator 460.

The second amplifying gain portion 480 is disposed along the signal line402 optically downstream of the second 980-1580 multiplexer 470.Although the amplifying gain portions 440, 480 do not have a minimum ormaximum length, those skilled in the art will recognize that the lengthsof the amplifying gain portions 440, 480 can be varied, in conjunctionwith different output powers of the pump laser 434, to provide differentamplification gains and/or output powers.

A third optical isolator 490 is disposed along the signal line 402optically downstream of the second C-L band multiplexer 426. The thirdoptical isolator 490 prevents backscattered light and other opticalnoise from traveling backward along the signal line 402, from the thirdoptical isolator 490 toward the second amplifying gain portion 480. Thesecond C-L band multiplexer 426 is optically disposed between the secondamplifying gain portion 480 and the third optical isolator 490. Thethird optical isolator 490 is optically connected to the output P_(out)of the amplifier 400.

In operation, the signal light λ_(S) having a wavelength band ofapproximately between 1565 and 1625 nanometers is injected into theamplifier 400 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 402 to the first opticalisolator 410. The signal light λ_(S) passes through the first opticalisolator 410 and along the signal line 402 to the first C-L bandmultiplexer 420. The signal light λ_(S) passes through the first C-Lband multiplexer 420 to the pump-signal multiplexer 430.

The pump laser 434 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 432 to the pump-signal multiplexer 430. At thepump-signal multiplexer 430, the signal light λ_(S) is combined with thepump signal λ_(P) emitted by the pump laser 434. The combined signallight λ_(S) and the pump signal λ_(P) are transmitted to the firstamplifying gain portion 440. The pump signal λ_(P) excites the rareearth element in the first amplifying gain portion 440, amplifying thesignal light λ_(S).

However, as the signal light λ_(S) is transmitted through the firstamplifying gain portion 440, first ASE, which travels in both forwardand backward directions relative to the signal light λ_(S), isgenerated. Only backward ASE will be discussed. The first ASE travels ina second, opposite direction from the signal light λ_(S), toward theinput P_(in). The first ASE travels through the pump-signal multiplexer430 and to the first C-L band multiplexer 420. At the first C-L bandmultiplexer 420, any L band light is directed along the signal line 402to the first optical isolator 410, which blocks further transmission ofthe L band light toward the input P_(in), while C band light in the formof first ASE is directed along the ASE guide 422 to the second C-L bandmultiplexer 426.

The signal light λ_(S), now amplified by the first amplifying gainportion 440, is transmitted through the first 980-1580 multiplexer 450,the second optical isolator 460, and the second 980-1580 multiplexer470. Pump light λ_(P) which exits the first amplifying gain portion 440is diverted by the first 980-1580 multiplexer 450 to the bypass guide452. The pump light λ_(P) is transmitted through the bypass guide 452 tothe second 980-1580 multiplexer 470, where the pump light λ_(P) isrecombined with the signal light λ_(S). The bypass guide 452 isinstalled between the first and second amplifying gain portions 440, 480to eliminate any high insertion loss which may occur if the pump lightλ_(P) is directed through the second optical isolator 460.

The combined pump light λ_(P) and signal light λ_(S) are thentransmitted to the second amplifying gain portion 480. The pump signalλ_(P) excites the rare earth element in the second amplifying gainportion 480, further amplifying the signal light λ_(S). The first ASEtravels from the second C-L band multiplexer 426 through the secondamplifying gain portion 480, where the first ASE acts to counter-pumpthe signal light λ_(S) to provide additional amplification of the signallight λ_(S). As a further benefit, the counter-pumping of the secondamplifying gain portion 480 eliminates residual pump signal λ_(P) at theoutput P_(out), The signal light λ_(S) that has been amplified in thesecond amplifying gain portion 480 is then transmitted along the signalline 402 to the third optical isolator 490 and to the output P_(out).

However, as the signal light λ_(S) is transmitted through the secondamplifying gain portion 480, second ASE, which travels in both forwardand backward directions relative to the signal light λ_(S), isgenerated. Only backward ASE will be discussed. The second ASE travelsin the second, opposite direction from the signal light λ_(S), towardthe input P_(in). The second ASE, as well as any residual first ASEwhich is transmitted from the second amplifying gain portion 480 towardthe input P_(in), is absorbed by the second optical isolator 460 toprevent the possibility of lasing. After amplification by the pump laser434 and by the ASE, the signal light λ_(S) has an amplified intensity,larger than the initial intensity.

A fifth embodiment of an amplifier 500 according to the presentinvention is shown in FIG. 5. The amplifier 500 is similar to theamplifier 400, with the exception that, in the amplifier 500, a filter524 is optically disposed along the ASE guide 422 between the first andsecond C-L band multiplexers 420, 426. Preferably, the filter 524 is aBragg grating, a flat connector, or other optical filter known in theart. The filter 524 reflects a small portion (approximately 4%) of thefirst ASE and allows the remainder (approximately 96%) to be transmittedto the second C-L band multiplexer 426 as described above. The reflectedASE enters the signal line 402 at the first C-L band multiplexer 420 andis transmitted along the signal line 402 to the first amplifying gainportion 440, where the reflected ASE provides additional pumping powerto the signal light λ_(S) in the first amplifying gain portion 440.Further operation of the amplifier 500 is as described above withreference to the amplifier 400. After amplification by the pump laser434 and by the ASE, the signal light λ_(S) has an amplified intensity,larger than the initial intensity.

A first version of a sixth embodiment of an L band amplifier 600according to the present invention is shown schematically in FIG. 6A.The amplifier 600 includes a signal line 602 which extends from an inputP_(in) at one end of the amplifier 600 to an output P_(out) at anotherend of the amplifier 600. The input P_(in) and the output P_(out) areoptically connected to each other along the signal line 602 through theamplifier 600. Signal light λ_(S) having at least one, and preferably,multiple wavelengths is transmitted through the amplifier 600 from theinput P_(in) to the output P_(out), from left to right as shown in FIG.6A. The wavelengths of the signal light λ_(S) preferably rangeapproximately from 1565 to 1625 nanometers, placing the signal lightλ_(S) in the L band.

A first optical isolator 610 is optically disposed in the signal line602 between the input P_(in) and the output P_(out). The first opticalisolator 610 prevents backscattered light and other optical noise fromtraveling backward along the signal line 602, from the first opticalisolator 610 toward the input P_(in). A Bragg grating 620 is disposedalong the signal line 602 optically downstream of the first opticalisolator 610. Preferably, the Bragg grating is a fiber Bragg grating,although the Bragg grating can be other types of reflective elements,including, but not limited to, waveguide Bragg gratings.

A pump-signal multiplexer 630 is disposed along the signal line 602optically downstream of the Bragg grating 620. The pump-signalmultiplexer 630 couples a first end of a pump laser guide 632 to thesignal line 602. A second end of the pump laser guide 632 is connectedto a pump laser 634. Preferably, the pump laser 634 is a 980 nanometerlaser which emits a pump signal λ_(P), although those skilled in the artwill recognize that other wavelengths can be used as well. Alsopreferably, the pump laser 634 has an output power of at least 100 mW,although those skilled in the art will recognize that the pump laser 634can have other output powers as well.

A rare earth doped amplifying gain medium 640 is disposed along thesignal line 602 optically downstream of the pump-signal multiplexer 630.A second optical isolator 650 is disposed along the signal line 602optically downstream of the amplifying gain medium 640. The secondoptical isolator 650 prevents backscattered light and other opticalnoise from traveling backward along the signal line 602, from the secondoptical isolator 650 toward the amplifying gain portion 640. The secondoptical isolator 650 is optically connected to the output P_(out) of theamplifier 600.

In operation, the signal light λ_(S) having a wavelength band ofapproximately between 1565 and 1625 nanometers is injected into theamplifier 600 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 602 to the first opticalisolator 610. The signal light λ_(S) passes through the first opticalisolator 610 and along the signal line 602 to the Bragg grating 620. Thesignal light λ_(S) passes through the Bragg grating 620 to thepump-signal multiplexer 630.

The pump laser 634 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 632 to the pump-signal multiplexer 630. At thepump-signal multiplexer 630, the signal light λ_(S) is combined with thepump signal λ_(P) emitted by the pump laser 634. The combined signallight λ_(S) and the pump signal λ_(P) are transmitted to the amplifyinggain medium 640. The pump signal λ_(P) excites the rare earth element inthe amplifying gain medium 640, amplifying the signal light λ_(S).

However, as the signal light λ_(S) is transmitted through the amplifyinggain medium 640, ASE, which travels in both forward and backwarddirections relative to the signal light λ_(S), is generated. Onlybackward ASE will be discussed. The ASE travels in a second, oppositedirection from the signal light λ_(S), toward the input P_(in). The ASEtravels through the pump-signal multiplexer 630 and to the Bragg grating620.

The Bragg grating 620 reflects the ASE in a narrow band of approximately0.3 to 4 nm between approximately 1525 and 1560 nm. Remaining ASE isallowed to travel backward toward the first optical isolator 610.Although the narrow band of 0.3 to 4 nm is preferred, those skilled inthe art will recognize that a wider band can be reflected.

The reflected ASE combines with the signal light λ_(S) at the Bragggrating 620 and is transmitted toward the pump-signal multiplexer 630.At the pump-signal multiplexer 630, the ASE and the light signal λ_(S)combine with the pump signal λ_(P). The reflected ASE acts as a C bandpump seed to suppress the backward ASE. The reflected ASE absorbs muchof the pump signal λ_(P) from the pump laser 634, thus preventing thebackward ASE from absorbing as much pump signal λ_(P) from the pumplaser 634. The reflected ASE is amplified by the gain medium 640, whichis first pumped by the pump laser 634. The amplified ASE then serves asa pump for the light signal λ_(S). The reflected ASE then imparts asubstantial portion of its energy to the signal light λ_(S) in theamplifying gain medium 640. After amplification by the pump laser 634and the reflected ASE, the signal light λ_(S) has an amplifiedintensity, larger than the initial intensity. The amplified signal lightλ_(S) is then transmitted from the amplifying gain medium 640, throughthe second optical isolator 650, and to the output P_(out).

A second version of the sixth embodiment of the L band amplifier 600′ isshown schematically in FIG. 6B. The amplifier 600′ is similar to theamplifier 600 as described above, but instead of one Bragg grating 620optically disposed between the first optical isolator 610 and thepump-signal multiplexer 630, a plurality of Bragg gratings 620, 622, 624are optically disposed between the first optical isolator 610 and thepump-signal multiplexer 630. Each Bragg grating 620, 622, 624 is tunedto reflect different wavelength bands, resulting in increased reflectedASE at the reflected wavelengths. The reflected ASE acts as a C bandpump seed to suppress the backward ASE. The reflected ASE absorbs muchof the pump signal λ_(P) from the pump laser 634, thus preventing thebackward ASE from absorbing as much pump signal λ_(P) from the pumplaser 634. The reflected ASE is amplified in the amplifying gain medium640 and then serves as a pump for the light signal λ_(S). The reflectedASE then imparts a substantial portion of its energy to the signal lightλ_(S) in the amplifying gain medium 640. Although three Bragg gratings620, 622, and 624 are shown, those skilled in the art will recognizethat more or less than three Bragg gratings can be used.

A third version of the sixth embodiment of the L band amplifier 600″ isshown schematically in FIG. 6C. The amplifier 600″ is similar to theamplifier 600′ as described above, but in addition to the Bragg gratings620, 622, and 624 which are optically disposed upstream of the gainmedium 640, additional Bragg gratings 626, 628 are optically disposeddownstream of the amplifying gain medium 640, with a second amplifyinggain medium 642 optically disposed between the Bragg gratings 626, 628and the second optical isolator 650. However, those skilled in the artwill recognize that the Bragg gratings 626, 628 can be opticallydisposed along the signal line 602 anywhere between the Bragg grating624 and the second optical isolator 650.

In operation, the signal light λ_(S) having a wavelength band ofapproximately between 1565 and 1625 nanometers is injected into theamplifier 600 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 602 to the first opticalisolator 610. The signal light λ_(S) passes through the first opticalisolator 610 and along the signal line 602 to the Bragg gratings 620,622, 624. The signal light λ_(S) passes through the Bragg gratings 620,622, 624 to the pump-signal multiplexer 630.

The pump laser 634 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 632 to the pump-signal multiplexer 630. At thepump-signal multiplexer 630, the signal light λ_(S) is combined with thepump signal λ_(P) emitted by the pump laser 634. The combined signallight λ_(S) and the pump signal λ_(P) are transmitted to the amplifyinggain medium 640. The pump signal λ_(P) excites the rare earth element inthe amplifying gain medium 640, amplifying the signal light λ_(S).

However, as the signal light λ_(S) is transmitted through the amplifyinggain medium 640, ASE, which travels in both forward and backwarddirections relative to the signal light λ_(S), is generated. Onlybackward ASE will be discussed. The ASE travels in a second, oppositedirection from the signal light λ_(S), toward the input P_(in). The ASEtravels through the pump-signal multiplexer 630 and to the Bragggratings 620, 622, 624. Preferably, the Bragg gratings are selected toreflect ASE in the range of approximately 1535 to 1560 nm, so that ASEof the selected wavelengths is reflected back into the amplifying gainmedium 640 as seed to amplify the signal light λ_(S) betweenapproximately 1565 nm and 1580 nm.

As the signal light λ_(S) and any remaining pump light λ_(P) is furthertransmitted along the signal line 602, the signal light λ_(S) and thepump light λ_(P) pass through the Bragg gratings 626, 628 to the secondamplifying gain medium 642, where the pump light λ_(P) excites rareearth element in the second amplifying gain medium 642, amplifying thesignal light λ_(S).

However, as the signal light As is transmitted through the secondamplifying gain medium 642, additional backward ASE is generated. TheASE travels in a second, opposite direction from the signal light λ_(S),toward the input P_(in). The ASE travels to the Bragg gratings 626, 628.Preferably, the Bragg gratings are selected to reflect ASE in the rangeof approximately 1560 to 1580 nm, so that ASE of the selectedwavelengths is reflected back into the second amplifying gain medium 642as seed to amplify the signal light λ_(S) between approximately 1580 nmand 1625 nm.

As is known by those skilled in the art, longer lengths of theamplifying gain media 640, 642 provide for higher emissions at longerwavelengths. The first amplifying gain medium 640 amplifies the signallight λ_(S) in a shorter L band region of approximately between 1565 and1580 nm, while the second amplifying gain medium 642 amplifies thesignal light λ_(S) in a longer L band region of approximately between1560 and 1580 nm. Preferably, the lengths of each of the amplifying gainmedia 640, 642 are each approximately 80 to 100 meters. However, higherrare earth concentrations in the amplifying gain media 640, 642 willallow comparable amplification of the light signal at shorter lengths,such as approximately 60 meters each. If desired, the lengths of each ofthe amplifying gain media 640, 642 can be optimized to provide maximumamplification within predetermined bandwidths. For such an arrangement,it is possible that the length of the amplifying gain medium 640 can bezero; in other words, the amplifying gain medium 640 can be omitted andthe second amplifying gain medium 642 can be the only amplifying gainmedium in the amplifier 600″.

Also, referring to FIGS. 6A, 6B, and 6C, although the Bragg gratings620, 622, and 624 are optically disposed between the first opticalisolator 610 and the pump-signal multiplexer 630, those skilled in theart will recognize that the Bragg gratings 620, 622, and 624 can beoptically disposed between the first optical isolator 610 and the secondoptical isolator 650.

A first version of a seventh embodiment of an L band amplifier 700according to the present invention is shown schematically in FIG. 7A.The amplifier 700 includes a signal line 702 which extends from an inputP_(in) at one end of the amplifier 700 to an output P_(out) at anotherend of the amplifier 700. The input Pi, and the output P_(out) areoptically connected to each other along the signal line 702 through theamplifier 700. Signal light λ_(S) having at least one, and preferably,multiple wavelengths is transmitted through the amplifier 700 from theinput P_(in) to the output P_(out), from left to right as shown in FIG.7A. The wavelengths of the signal light λ_(S) preferably rangeapproximately from 1565 to 1625 nanometers, placing the signal lightλ_(S) in the L band.

A C-L band multiplexer 710 is optically disposed in the signal line 702between the input P_(in) and the output P_(out). The C-L bandmultiplexer 710 optically connects a tunable C band seed pump 712 to thesignal line 702 via a C band pump guide 714. Alternatively, an opticalcoupler (not shown) can be used instead of the C-L band multiplexer 710.A first optical isolator 720 is disposed in the signal line 702optically downstream of the C-L band multiplexer 710. The first opticalisolator 720 prevents backscattered light and other optical noise fromtraveling backward along the signal line 702, from the first opticalisolator 710 toward the input P_(in).

A pump-signal multiplexer 730 is disposed along the signal line 702optically downstream of the first optical isolator 720. The pump-signalmultiplexer 730 couples a first end of a pump laser guide 732 to thesignal line 702. A second end of the pump laser guide 732 is connectedto a pump laser 734. Preferably, the pump laser 734 is a 980 nanometerlaser which emits a pump signal λ_(P), although those skilled in the artwill recognize that other wavelengths can be used as well. Alsopreferably, the pump laser 734 has an output power of at least 100 mW,although those skilled in the art will recognize that the pump laser 734can have other output powers as well.

A rare earth doped amplifying gain medium 740 is disposed along thesignal line 702 optically downstream of the pump-signal multiplexer 730.A second optical isolator 750 is disposed along the signal line 702optically downstream of the amplifying gain medium 740. The secondoptical isolator 750 prevents backscattered light and other opticalnoise from traveling backward along the signal line 702, from the secondoptical isolator 750 toward the amplifying gain portion 740. The secondoptical isolator 750 is optically connected to the output P_(out) of theamplifier 700.

In operation, the signal light λ_(S) having a wavelength band ofapproximately between 1565 and 1625 nanometers is injected into theamplifier 700 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 702 to C-L band multiplexer710. The signal light λ_(S) passes through the C-L band multiplexer 710and along the signal line 702 to the first optical isolator 720. Thesignal light λ_(S) passes through the first optical isolator 720 to thepump-signal multiplexer 730.

The C band seed pump 712 generates a tunable C band light signal λ_(C),between 1530 nm and 1570 nm. The C band seed pump 712 can be tuned togenerate an optimized C band seed wavelength for transmission toward theamplifying gain medium 740. The C band light signal λ_(S) travels alongthe C band pump guide 714 to the C-L band multiplexer 720, where the Cband light signal λ_(S) enters the signal line 702. The C band lightsignal λ_(S) then travels along the signal line 702 with the signallight λ_(S).

The pump laser 734 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 732 to the pump-signal multiplexer 730. At thepump-signal multiplexer 730, the signal light λ_(S) and the C band lightsignal λ_(C) are combined with the pump signal λ_(P) emitted by the pumplaser 734. The C band light signal λ_(C) is amplified in the gain medium740 and suppresses the backward ASE. The amplified C band light signalλ_(C), as well as the signal light λ_(S) and the pump signal λ_(P),propagate through the amplifying gain medium 740. The amplified C bandlight signal λ_(C) and any residual pump signal λ_(P) excite the rareearth element in the amplifying gain medium 740, amplifying the signallight λ_(S). The C band light signal λ_(C) does not significantlygenerate ASE in the C band because of the longer wavelength of the Cband light signal λ_(C). As a result, backward ASE is significantlyreduced and additional C band pumping by the C band seed is generated,resulting in greater amplification of the signal light λ_(S).

A second version of the seventh embodiment of the L band amplifier 700′is shown schematically in FIG. 7B. The second version is similar to thefirst version shown in FIG. 7A, but with additional C band seed pumps712 ₁ through 712 _(n) optically connected to the signal line 702. EachC band seed pump 712, 712 a through 712 ₁ generate C band seed at aseparate wavelength within the C band. The multiple wavelengths of Cband seed provide additional amplification of the signal light λ_(S)over the C band seed provided by the single C band seed pump 712.

An eighth embodiment of an L band amplifier 800 according to the presentinvention is shown schematically in FIG. 8. The amplifier 800 includes asignal line 802 which extends from an input P_(in) at one end of theamplifier 800 to an output P_(out) at another end of the amplifier 800.The input P_(in) and the output P_(out) are optically connected to eachother along the signal line 802 through the amplifier 800. Signal lightλ_(S) having at least one, and preferably, multiple wavelengths istransmitted through the amplifier 800 from the input P_(in) to theoutput P_(out), from left to right as shown in FIG. 8. The wavelengthsof the signal light λ_(S) preferably range approximately from 1565 to1625 nanometers, placing the signal light λ_(S) in the L band.

A first optical isolator 810 is optically disposed in the signal line802 between the input P_(in) and the output P_(out). The first opticalisolator 810 prevents backscattered light and other optical noise fromtraveling backward along the signal line 802, from the first opticalisolator 810 toward the input P_(in).

A pump-signal multiplexer 830 is disposed along the signal line 802optically downstream of the first optical isolator 810. The pump-signalmultiplexer 830 couples a first end of a pump laser guide 832 to thesignal line 802. A second end of the pump laser guide 832 is connectedto a pump laser 834. Preferably, the pump laser 834 is a 980 nanometerlaser which emits a pump signal λ_(P), although those skilled in the artwill recognize that other wavelengths can be used as well. Alsopreferably, the pump laser 834 has an output power of at least 100 mW,although those skilled in the art will recognize that the pump laser 834can have other output powers as well.

First and second Bragg gratings 840, 842 are disposed in the signal line802 optically downstream of the pump-signal multiplexer 830. A lasingmedium 844 is optically disposed between the first and second Bragggratings 840, 842. Preferably, the lasing medium 844 is a rare earthdoped fiber, although those skilled in the art will recognize that otherlasing media can be used. The Bragg gratings 840, 842 preferably reflectthe same wavelength of light, but with different percentages ofreflectability. Preferably, the first Bragg grating 840 reflects morelight than the second Bragg grating 842.

A rare earth doped amplifying gain medium 850 is disposed along thesignal line 802 optically downstream of the second Bragg grating 842. Asecond optical isolator 860 is disposed along the signal line 802optically downstream of the amplifying gain medium 850. The secondoptical isolator 860 prevents backscattered light and other opticalnoise from traveling backward along the signal line 802, from the secondoptical isolator 860 toward the amplifying gain portion 850. The secondoptical isolator 860 is optically connected to the output P_(out) of theamplifier 800.

In operation, the signal light λ_(S) having a wavelength band ofapproximately between 1565 and 1625 nanometers is injected into theamplifier 800 in a first direction at the input P_(in). The signal lightλ_(S) is transmitted along the signal line 802 to the first opticalisolator 810. The signal light λ_(S) passes through the first opticalisolator 810 and along the signal line 802 to the pump-signalmultiplexer 830.

The pump laser 834 transmits a 980 nanometer pump signal λ_(P) along thepump laser guide 832 to the pump-signal multiplexer 830. At thepump-signal multiplexer 830, the signal light λ_(S) is combined with thepump signal λ_(P) emitted by the pump laser 834. The combined signallight λ_(S) and the pump signal λ_(P) are transmitted through the firstand second Bragg gratings 840, 842 and the lasing medium 844 to theamplifying gain medium 850. The pump signal λ_(P) excites the rare earthelement in the amplifying gain medium 850, amplifying the signal lightλ_(S).

However, as the signal light λ_(S) is transmitted through the amplifyinggain medium 850, C band ASE, which travels in both forward and backwarddirections relative to the signal light λ_(S), is generated. Onlybackward ASE will be discussed. The ASE travels in a second, oppositedirection from the signal light λ_(S), toward the input P_(in). The ASEtravels through the second Bragg grating 842 and the lasing medium 844to the first Bragg grating 840. The ASE stimulates the rare earth ionsin the lasing medium 844, which in turn amplify the signal light λ_(S),as described above in previous embodiments of the present invention.

The first Bragg grating 840 reflects preferably approximately 99% of theASE in a narrow band of preferably approximately 1 nm betweenapproximately 1525 and 1560 nm. Remaining ASE is allowed to travelbackward toward the first optical isolator 810. Although the narrow bandof approximately 1 nm is preferred, those skilled in the art willrecognize that a wider band can be used. The reflected ASE travels backthrough the lasing medium 844, further stimulating the rare earth ionsin the lasing medium 844. Preferably approximately 80% of the reflectedASE is re-reflected by the second Bragg grating 842 back toward thelasing medium 844, setting up a lasing effect, with a substantialportion of the ASE being reflected between the first and second Bragggratings 840, 842.

The lasing medium 844 provides relatively high power (preferably up tobetween 6 and 8 mW) seed signal to the amplifier gain medium 850 tosuppress the backward ASE and to serve as a secondary pump for theamplifier 800. The amplified signal light λ_(S) is then transmitted fromthe amplifying gain medium 850, through the second optical isolator 860,and to the output

Although the embodiments described above are generally referred to ashaving several individual components, those skilled in the art willrecognize that components such as amplifying media, optical isolators,multiplexers, Bragg gratings, bypass guides, and ASE guides can beincorporated into a single or several planar waveguides.

The top two curves on the graph of FIG. 9 (solid square and solidcircle) show measured gain vs. input signal wavelength for the seventhembodiment of the present invention. The third curve (open circle) showsmeasured gain vs. input signal wavelength for the sixth embodiment ofthe present invention having only one Bragg grating 620, with the Bragggrating 620 reflecting approximately 25% of the ASE. The fourth curve(solid triangle) shows measured gain vs. input signal wavelength withoutany seed. The pump laser 634 used was a 980 nm pump, operating atapproximately 180 mW.

FIG. 10 shows calculated gain vs. amplifying medium length at variouswavelengths of a seed signal at 0 dBm. FIG. 11 shows calculated gain vs.amplifying medium length at various 1560 nm seed powers for a 1600nm,−30 dBm signal. FIG. 12 shows calculated noise figure vs. amplifyingmedium length for a signal with and without seed. The calculations wereperformed using OPTIWAVE® software. The calculations show significantgain with relatively low noise figures at particular lengths of theamplifying gain medium 850 for various seed wavelengths and various seedpowers. The seed is generated according to known methods, but preferablyusing Bragg gratings as shown in the sixth embodiment or using a seedlaser as shown in the seventh embodiment or building a Bragg gratinglaser optically upstream of the amplifier gain medium as shown in theeighth embodiment.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

What is claimed is:
 1. An L band optical amplifier comprising: a signalline including: an input; an output disposed optically downstream of theinput; and an amplifying gain medium optically disposed between theinput and the output; a laser optically connected to the amplifying gainmedium; and means for directing C band light into the amplifying gainmedium, wherein the means comprises at least first and second reflectiveelements optically disposed in the signal line between the input and theamplifying gain medium, wherein the at least first reflective elementreflects a first bandwidth of C band light generated in the amplifyinggain medium traveling in a second direction, opposite the firstdirection and the at least second reflective element reflects a secondbandwidth of C band light generated in the amplifying gain mediumtraveling in a second direction, opposite the first direction.
 2. The Lband optical amplifier according to claim 1, wherein the at least onereflective element reflects light within a range of approximately 1535to 1560 nm.
 3. The L band optical amplifier according to claim 1,further comprising a lasing medium optical disposed between the firstreflective element and the second reflective element.
 4. The L bandoptical amplifier according to claim 1, wherein the signal line furthercomprises an optical isolator optically disposed between the input andthe amplifying gain medium.
 5. The L band optical amplifier according toclaim 1, wherein the means for directing amplified spontaneous emissionincreases signal gain by approximately 14 dB.
 6. The L band opticalamplifier according to claim 1, wherein the laser is a 980 nm laser. 7.The L band amplifier according to claim 1, wherein the amplifying gainmedium comprises a first amplifying gain medium and a second amplifyinggain medium.
 8. The L band optical amplifier according to claim 7,wherein the signal line further comprises an optical isolator opticallydisposed between the amplifying gain medium and the output.
 9. The Lband amplifer according to claim 8, further comprising at least a secondreflective element optically disposed in the signal line between the atleast one reflective element and the optical isolator.
 10. The L bandamplifer according to claim 9, wherein the at least second reflectiveelement reflects light within a range of approximately 1560 to 1580 nm.11. The L band optical amplifier according to claim 1, wherein thelimited portion of C band light comprises between a bandwidth of betweenapproximately 0.3 and 4 nanometers of the C band light.
 12. The L bandamplifier according to claim 7, wherein the second amplifying gainmedium is optically disposed between the first amplifying gain mediumand the output and a second reflector is optically disposed between thefirst amplifying gain medium and the second amplifying gain medium, andwherein the second reflector reflects a limited portion of C band lighttraveling in a second direction, opposite the first direction.